1. Field of the Invention
The present invention relates to lamps with reflectors and image projection apparatuses. In particular, the present invention relates to high pressure mercury lamps used as light sources for projectors or the like and having relatively large amounts of mercury enclosed.
2. Description of the Related Art
In recent years, image projection apparatuses such as a liquid crystal projector and a DMD™ (Digital Micromirror Device) projector have been widely used as systems for realizing large-scale video images. For such an image projection apparatus, in general, a high pressure mercury lamp has been commonly used which is disclosed, for example, in Japanese Unexamined Patent Publication No. 2-148561.
FIG. 1 shows the construction of a high pressure mercury lamp disclosed in Japanese Unexamined Patent Publication No. 2-148561. The lamp 1000 shown in FIG. 1 is composed of a luminous bulb 1 mainly made of quartz and a pair of side tube portions (sealing portions) 2 extending from both sides of the luminous bulb 1. In each of the side tube portions 2, an electrode structure made of metal is embedded, whereby power can be supplied from the outside to the luminous bulb 1. The electrode structure has an electrode 3 of tungsten (W), a molybdenum (Mo) foil 4, and an external lead 5, which are electrically connected in the listed order. A coil 12 is wound around the tip of the electrode 3. The luminous bulb 1 encloses mercury (Hg) and argon (Ar) as luminous species, and a smaller amount of halogen gas (not shown).
The principle of operation of the lamp 1000 will be described briefly. When a starting voltage is applied to respective ends of the pair of external leads 5, Ar discharge occurs and the temperature within the luminous bulb 1 is raised. This temperature rise evaporates Hg atoms, and the evaporated atoms in gaseous form fill the inside of the luminous bulb 1. Hg between both the electrodes 3 is exited by electrons emitted from one of the electrodes 3, and then becomes luminescent. Therefore, as the vapor pressure of Hg serving as a luminous species is higher, light with higher intensity is emitted. Moreover, as the vapor pressure of Hg is higher, the potential difference (the voltage) between the electrodes increases. Therefore, when lamps are operated at the same rated power, current flowing in a lamp with a higher Hg vapor pressure can be lower than that with a lower Hg vapor pressure. This means that a load on the electrode 3 can be lightened, which contributes to life extension of the lamp. Consequently, as the Hg vapor pressure is higher, a lamp with more excellent property in intensity and durability can be provided.